In a cellular wireless network, individual base stations define network cells. Mobile devices wirelessly connect to these base stations and transfer connections between cells by handing over sessions to different base stations in the network. When connecting to cells on these networks, mobile devices rely on standards for setting up and managing connections. Such standards include the Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), Interim Standard 95 (IS-95), and Long Term Evolution (LTE). The Third Generation Partnership Project (3GPP) has defined the standards for LTE in 3GPP documents TS 36.101-TR 36.956. In particular, the air interface is referred to as the Evolved Universal Terrestrial Radio Access (E-UTRA). Mobile devices implementing the LTE standards are also backward compatible with earlier air interfaces, including UMTS Terrestrial Radio Access (UTRA) and GSM EDGE Radio Access Network (GERAN).
A mobile device that connects to an LTE network through a base station, often called a Node-B or eNode-B, uses an autonomous search function (ASF) in various situations. ASFs are functions that modify their own functionality in response to external inputs, optimizing search strategy to the environment. Mobile devices use ASFs on an LTE network to scan a range of frequencies to find information broadcast by cells. Information broadcast by UTRA and E-UTRA cells may include the cell's closed subscriber group (CSG) ID, information specifying supported protocols, information specifying supported radio access technology (RAT), and the rating of the cell within its frequency, as well as other information identifying the cell and how to connect to it.
The mobile device may use information about detected cells to initiate proximity detection messages for call handover or handover avoidance. When a mobile device enters or leaves the proximity of another cell while connected to a UTRA or E-UTRA base station, the network may initiate handover messages between the mobile device's current base station and the detected cell's base station. This allows the base station to transfer the mobile device's session between cells without a connectivity lapse. The mobile device sends proximity messages to help the network determine when handover procedures are necessary.
The mobile device may also use an ASF to find cells to camp on in idle mode. A mobile device camps on a cell when the mobile device does not have an active session with a cell but is monitoring cell system information and in most cases paging information. When the mobile device finds closed cells that are on its CSG whitelist, the mobile device will select the CSG cell for camping over normal cells. Closed cells are cells that are not for general use but are dedicated to a particular group of users. One type of closed cell is a femtocell, which is a low-power cell deployed in a subscriber's home to extend connectivity through a broadband Internet connection. A mobile device's CSG whitelist is a list of closed cells it may access. This whitelist may be sent to the mobile device by upper layers of the network. The mobile device may also maintain and update the list upon successfully connecting to a closed cell.